1. Field of the Invention
This invention relates generally to high frequency demodulating circuits used in communications applications and, more particularly, to a high frequency demodulating circuit in which the necessary signal processing functions of mixing, filtering, and amplification are combined to minimize cost.
2. Discussion of the Related Art
Frequency down-conversion mixers are generally implemented using a prior art circuit called a "Gilbert cell" shown in FIG. 1. The Gilbert cell is a fully differential architecture whose input is a set of differential voltages V.sub.LO.+-. and V.sub.RF.+-. that generates a differential voltage output V.sub.IF.+-.. The voltage output may be tuned or filtered to remove unwanted by-products generated by the mixing process. In a demodulator the wanted output is generally referred to as the baseband output. Its frequency is the difference of the frequencies at the LO (local oscillator) and RF (radio frequency) inputs.
One of the limitations of the basic Gilbert cell is that it is difficult to achieve a large voltage swing for a given supply voltage. Since the wanted and unwanted components are present in equal measure the available swing range is wasted. The same criterion also limits the voltage gain available in the mixing process. Larger voltage gain implies larger voltage swing at the outputs. If the voltage swing is limited the practical voltage gain is also limited. Another condition limiting the voltage gain comes from the DC biasing considerations. The gain is set by the ratio of resistors R.sub.L /R.sub.IN. In order to achieve a specific distortion performance level, the resistor R.sub.IN is chosen to be a minimum value for a given bias current I.sub.B. Choosing large gain values implies a large value for the resistor R.sub.L. For practical operation, the DC values at the V.sub.IF.+-. nodes cannot be lower than the DC values at the V.sub.LO.+-. nodes.
The above limitations are generally overcome by keeping the gain low in the mixers and using filters situated externally to the integrated circuit for removing the unwanted high frequency components. This means that the signals must first be routed out of the integrated circuit, filtered, then routed back to the integrated circuit for further amplification. This solution requires more pins for taking the signals out of the integrated circuit and back into the integrated circuit. The requirement to take the signals out and back in to the integrated circuit degrades the signals. In addition, it is difficult to maintain the balanced differential nature of the signals once they are taken out of the integrated circuit and then reinserted into the integrated circuit. If the differential nature of the signals is not fully maintained the succeeding amplifiers will amplify not only the desired baseband signal but will amplify the extra distortion introduced externally as well.
What is needed is a differential amplifier circuit that maintains the fully differential nature of the signals, minimizes distortion, and provides the required high gain.